It has been believed for some time now that the current magnetic recording media will at some density become unusable on account of supraparamagnetism. Supraparamagnetism is a thermal instability of the magnetization in an unpatterned magnetic recording medium which is projected to occur as the sizes of the magnetic domains approach the sizes of the magnetic metal grains. Because of this belief, there has been a good deal of research into different ways of overcoming the supraparamagnetic limit by making patterned magnetic recording media. See, e.g., C. A. Ross, “Patterned Magnetic Media,” Annual Rev. Mater. Res. 31:203-35 (2001). Furthermore, patterned media offer less magnetic recording noise at the same recording density point.
In more detail, as we scale continuous media to smaller bit (or magnetic domain) dimensions, we also have to scale grain sizes, because signal to noise ratio is roughly proportional to the number of grains. At some point, those grain sizes become so small, that the thermal energy alone is sufficient to flip the bit, and the media becomes unstable. The critical grain volume Vg that determines the onset of supraparamagnetic limit is determined by the condition that the stored magnetic energy KuVg is about 40-60 times larger than the thermal energy kBT, where ku and kB are the magnetic anisotropy and Boltzmann constant, and T is the temperature. When bit densities are high enough that grain sizes in unpatterned media fall below the critical grain volume, patterned media is preferred since it offers one relatively large island of magnetic material, acting as a single magnetic domain, and therefore has an improved signal to noise ratio. It is believed that recourse to patterned media may become necessary at recording densities of very roughly 500 Gb/in2 to 1000 Gb/in2.
Possible approaches to achieving densities of 500 Gb/in2 include perpendicular recording and thermally assisted writing on high coercivity media. However, these approaches have not yet been demonstrated to be viable for data storage densities on the order of 500 Gb/in2.
Perpendicular recording refers to data recording on a hard disk in which the poles of the magnetic bits on the disk are aligned perpendicularly to the surface of the disk platter. Perpendicular recording can deliver up to 10 times the storage density of longitudinal recording, on the same recording media. Current hard disk technology with longitudinal recording has an estimated limit of 100-150 Gb/in2 due to the superparamagnetic effect. As discussed above, it is estimated that, when the bits are of the size required to achieve densities above that limit, the grain size becomes so small that thermal energy alone can flip its magnetization direction. This would cause random data corruption which would be unacceptable in practice. Perpendicular recording gets around the supraparamagnetic limit by re-aligning the poles of the bits perpendicularly to the surface of the disk so they can be placed closer together on the platter, thus increasing storage density by a factor of 10.
U.S. Pat. Nos. 6,313,969, 6,420,058, and 6,440,520, for example, teach methods of making patterned magnetic media.
An important issue with patterned magnetic media is the level of perfection associated with the planarization of the disk. For example, the read/write head of a data storage device must be able to fly over the face of a disk rotating at speeds ranging from 3600 up to 15000 rpm, to read or write on concentric data tracks disposed on the surface of the disk. The spacing between read/write head and disk surface as the head flies over that surface is measured in nanometers, thus requiring minimal anomalies in the disk surface to assure smooth flying for the read/write head. A preferred approach to minimizing anomalies in the media used in a data storage device is to planarize the patterned media.
There is consequently a need in the art for a method of making patterned magnetic recording media which allows good planarization as a part of the manufacturing process.